The present invention is directed to processes for forming beads of polystyrene methyl methacrylate.
Styrenic polymers have a wide variety of applications, including the formation of expanded polystyrene which can be used to make a variety of products. Processes for forming styrenic polymers include emulsion polymerization, suspension polymerization, and the use of particular suspensions or emulsion aids.
Polymer particles are useful in applications such as the formation of expanded resins, for example, expanded polystyrene. Expanded polystyrene and other expanded resins can be prepared from expandable polymeric particles made by contacting the polymeric particles with a volatile compound known as a xe2x80x9cblowing agentxe2x80x9d or xe2x80x9cexpanding agentxe2x80x9d. Such agents include aliphatic hydrocarbons such as butane, pentanes, hexanes, and halogenated hydrocarbons such as trichloromethane, trichlorofluoromethane, and dichloromethane. The particles in contact with the expanding agent may be expanded by heating, or by exposure to reduced pressure as in a vacuum. The size and size distribution of the expanded particles will depend upon the size and size distribution of the expandable particles.
Expanded and expandable polymeric resins have applications in packaging, consumer products, and in materials processing. Examples of materials processing applications for expanded polymeric resins include so-called xe2x80x9clost foam castingxe2x80x9d, also called xe2x80x9cevaporative pattern castingxe2x80x9d. In lost foam casting, molten metal is poured into a pattern made of expanded polymeric material, i.e. a foam, coated with a refractory material surrounded and supported by unbounded sand. The foam is decomposed by the heat of the molten metal and replaced by the metal.
However, parts cast of metals such as iron, using expanded polystyrene foams, may have an unacceptable amount of surface defects, such as lustrous carbon. Expandable resin compositions made of styrene-acrylic copolymers are described in U.S. Pat. No. 5,403,866.
Polymers such as polystyrene are frequently manufactured by water suspension. Some of the advantages of using water suspension are good temperature control, production of polymer with desired particle size, ease of discharging from the reactor, less expensive reactors, etc. Due to the development of the lost foam market, a need for the production of polystyrene methyl methacrylate copolymer arises.
Copolymer production poses several challenges as compared to the polystyrene production. Polystyrene suspension based on tricalcium phosphate (TCP) and potassium persulfate count on slight styrene water solubility. The styrene in the water phase reacts with the potassium persulfate and forms a surfactant. This surfactant is needed in maintaining the suspension stability. The production of the copolymer calls for a mixture of styrene and methyl methacrylate monomers. The methyl methacrylate is more polar, and thus a more hydrophilic compound. Therefore, the methyl methacrylate concentration in the water phase will be higher than the styrene concentration; more methyl methacrylate than styrene will be available to react with initiators such as potassium persulfate. The resulting surfactant will have a chain of predominantly methyl methacrylate. If the surfactant has a fairly low molecular weight, it will be water soluble and it may not be sufficient to lower the surface tension to the extent to provide a stable suspension. On the other hand, surfactants such as sodium alkyl benzene sulfonate have a long aliphatic hydrophobic chain that may not interact with the more hydrophilic surface of the beads. A balanced mixture of potassium persulfate and sodium alkyl benzene sulfonate can provide a detergent mixture that better matches the copolymer""s surface polarity.
The viscosity of the organic phase copolymers increases relatively slowly in the beginning, followed by a rapid increase in the viscosity. The rapid increase in the viscosity is accompanied by a well pronounced exothermic reaction. The exotherm is more pronounced when the temperature of the reaction is higher.
When polystyrene is polymerized by itself, the increase of the organic phase viscosity is more gradual and the exothermic energy released by the polymerization is easier to dissipate, even at relatively high temperatures.
All of the above mentioned difficulties make the production of usable size (200 to 1000 micrometers) beads of polystyrene methyl methacrylate in commercial size reactors a challenge.
The present invention relates to a range of suspensions and conditions that produce beads with from about a 50:50 to 25:75 ratio of polystyrene/methyl methacrylate. The temperature of polymerization is lower than what most commercial production of polystyrene utilizes, in order to better dissipate the higher amount of energy released by the process. The preferred temperature of polymerization is from 60 to 80xc2x0 C.; a higher water to styrene ratio is used.
The ratio of water to styrene in the present invention is from about 55:45 to 70:30 parts water to styrene by weight. The suspension uses higher amounts of tricalcium phosphate (TCP) as compared with polystyrene suspensions. The useful range of TCP amounts is about 0.3 to 1.0% of TCP w/w based on water.
Although potassium persulfate can be used as a sole extender for 50:50 styrene to methyl methacrylate, increasing the amount of the methyl methacrylate calls for the use of a mixture of potassium persulfate and sodium alkyl benzene sulfonate.
Bead size can be controlled by varying the amount of TCP, extenders and agitation rate. Preferably, the extender amounts and the agitation speed are used for particle size control and most preferably the agitation speed is used to control the bead particle size.
Described below are experiments that provided a robust suspension system for higher amounts of methyl methacrylate to styrene ratios.
A glass six-liter reactor was used in testing, having a domed top and bottom with a jacket on the bottom and the sides. The reactor had a diameter of 200 mm and a depth of 280 mm. The temperature was maintained by using a water bath circulating the water through the reactor jacket. The variables considered were the ratio of styrene to methyl methacrylate, temperature, level of TCP, level and types of extending agents, and rate of agitation.
It was found that at a temperature as high as 90xc2x0 C., a methyl methacrylate to styrene ratio of 70:30 can cause the reaction speed to accelerate to levels where the cooling was not sufficient, even in the small reactor that had a favorable cooling area to volume ratio. This further increased the temperature and rate of reaction. The reaction temperature was then reduced to 80xc2x0 C. When lower ratios of methyl methacrylate to styrene such as 50:50 were used, it was possible to effectively carry the reaction and control the temperature at 90xc2x0 C.
The bead size of the product could be controlled by several factors. As can be seen from Examples 1 to 4, the reduction of TCP and potassium persulfate increased the size of the polymer beads. A further decrease of the TCP and potassium persulfate led to very unstable suspensions. Therefore, sodium alkyl benzene sulfonate was added to the suspension and other means of bead size control were used. Lowering the amount of the extender also reduced the average bead size of the polymer, as observed in Examples 5 to 7. Another factor that could control the bead size was the rate of agitation. Examples 8 to 10 depict reactions run from 1900 to 1400 RPMs, yielding average bead sizes from 191 to 565 micrometers. Using the lower agitation rate from 1400 to 1600 RPM gave rise to better bead distributions and more stable suspensions than at higher RPMs with less TCP and extenders. Therefore, the agitation rate is the most preferred tool in bead size control.